This invention relates to substrate cleansing, etching or otherwise processing and more particularly to a substrate bath for optimizing megasonic cleaning, etching or otherwise processing of substrates supported therein.
In the production and manufacture of electrical components, it is a recognized necessity to be able to clean, etch or otherwise process substrates to an extremely high degree of cleanliness or uniformity. Various cleaning, etching, or stripping processes may be applied to the substrates a number of times in conjunction with the manufacturing steps to remove particulates, predeposited layers or strip resist, and the like.
One cleaning process that is often employed involves ultrasonic cleansing; that is, the application of high amplitude ultrasonic energy to the substrates in a liquid bath. More specifically, the ultrasonic energy is generally, but not limited to, the range of 0.60-1.00 MHz, and the process is termed megasonic cleaning. The liquid bath may comprise deionized water, standard cleaning solvents, dilute HF, sulfuric, phosphoric, organic strip, or the like. The amplitude and the length of time of application of the sonic energy are generally well known in the prior art.
Substrates are typically processed in batches, and likewise are generally cleaned in batches. For example, it is known in the prior art to support 20-50 substrates in a holder immersed in a megasonic bath for cleaning purposes. The holder (hereinafter, substrate cassette) maintain the substrates in a parallel array in regular spacing. It is known that megasonic energy is highly directional, and typically tends to impinge on the substrates in the cassette in an uneven manner. That is, the structural components of the substrate cassette comprise obstacles that block direct line-of-sight application of the sonic energy to some portions of some of the substrates, thereby reducing the effectiveness of the megasonic cleaning, etching, or stripping process and ultimately leading to a reduction in yield of those substrates.
One approach to overcoming this problem involves rotation of the substrates in the megasonic bath to expose all surface areas to sonic energy in a more uniform manner. Ironically, this tactic requires a cassette that is larger and more intricate than the stationary substrate cassettes, thereby blocking more of the megasonic energy. Moreover, the edge supports of the rotating substrates create friction and abrasion of the substrate edge surfaces, increasing the possibility of damage to the substrates and generation of unwanted particulates as they are being cleaned or otherwise processed.
It is also possible to physically move the megasonic transducers (or substrates in relation to the transducers) in an attempt to eliminate the shadowed areas of the substrates in the cassette. This approach also leads to similar drawbacks and complexities that are not amenable to mass production and reliable results.
The present invention generally comprises a substrate bath that makes optimal use of megasonic energy to cleanse, etch or strip the entire surfaces of the substrate wafers. In general terms, the substrate bath includes a tank that is provided with reflecting surfaces that direct the megasonic energy to those portions of the substrates that would otherwise be sonically shadowed by the cassette that supports the substrates.
In one aspect, the invention includes a tank dimensioned to receive the substrate cassette, and a cleaning liquid that fills the tank and immerses the substrates. A megasonic transducer array and housing is supported beneath the bottom wall of the tank, and is acoustically coupled to the tank bottom by a mass of liquid such as water. The megasonic energy is radiated upwardly with very little divergence of the sound field. A pair of curved wall surfaces are formed with in the tank, each extending from one side wall to the bottom wall in curvilinear fashion and oriented longitudinally, the paired curved wall surfaces being disposed in laterally spaced, enantiomorphic relationship. The curved surfaces are arranged so that a significant amount of the megasonic energy impinges at an angle less than the critical angle, so that the energy is reflected in a diverging field that intersects the substrates and strikes those portions of the substrates that are shadowed by the cassette structure. Thus the megasonic cleaning, etching or processing is improved significantly.
In another aspect, the invention provides a method of carrying out a megasonic bath that includes radiating megasonic energy into a bath tank from one wall of the tank, and placing curved reflecting surfaces within the tank to diverge the sound field and obviate the sound-shadowing effect of a substrate cassette structure. The curved reflecting surfaces are preferably convex surfaces, and are placed so that they receive sonic energy from the primary source at an angle less than the critical angle, whereby reflection of a substantial portion of the sonic energy toward the substrates is achieved.